The magnesium isotopic compositions of the crust and mantle: A study on the Oman ophiolite

Magnesium isotope compositions (δ26Mg) can provide valuable insights into Mg cycling through surface fluid-rock interactions. Isotopic variations of Mg within the oceanic lithosphere imply incorporation of isotopically different Mg, but the underlying mechanisms remain uncertain. In this study, we measure the Mg isotopic composition of the oceanic crust (n = 32) and mantle peridotite (n = 13) from the Oman ophiolite, which has experienced magmatism at the spreading center and subduction zone. The crustal section records δ26Mg values that range from −0.58 to −0.04 ‰. In the upper crustal section, higher Mg-bearing mineral abundance tends to have lighter δ26Mg values. However, the lower crustal section has no correlation with the δ26Mg values, indicating that the hydrothermal Mg sink formed by seawater was significant within the upper 2 km of the oceanic crust. In the mantle section, the δ26Mg values range from −0.39 ‰ to −0.07 ‰. Seven peridotite samples, with a loss of MgO by up to 5.3%, have slightly higher δ26Mg values than the global mantle, which may be attributed to low-temperature weathering. In contrast, three peridotite samples with higher melting degrees (spinel Cr# = Cr/(Cr + Al) atomic ratio) than the adjacent rocks are considered to have been affected by subduction-related interactions, forming lighter δ26Mg values. The isotopically light Mg source was subducted sediments, but its low concentration of Mg requires a water/rock ratio >> 1. Therefore, we conclude that the contribution of slab-derived Mg and modification of mantle δ26Mg values were localized.